As wind energy business becomes more and more important, gearbox concepts are thoroughly studied nowadays. Known concepts of gearboxes for wind turbines may comprise at least one planetary gear unit and a parallel gear unit for connecting the planetary gear unit to the generator of the wind turbine.
FIG. 1 and FIG. 2 schematically illustrate a type of gearbox 1 for a wind turbine according to the prior art. In this example, the gearbox 1 comprises one planetary gear unit 2 and a two-stage parallel gear unit 3. The planetary gear unit 2 comprises a planet carrier 4 which supports a plurality of planet gears 5. The planetary gear unit 2 furthermore comprises a ring gear 6 and a sun gear 7. In the example given in FIG. 1 and FIG. 2 the two-stage parallel gear unit 3 comprises a low speed shaft 8, an intermediate shaft 9 and a high speed shaft 10, which are all parallel to each other and which are each rotatably supported by bearings 11. The parallel gear unit 3 furthermore comprises two gears 12, 13. The gears 12, 13 respectively mesh with pinion 14 on the intermediate shaft 9 and pinion 15 on the high speed shaft 10.
According to other known concepts, gearboxes 1 for wind turbines may comprise two planetary gear units 2 and a one-stage parallel gear unit 3. This is illustrated in FIG. 3. In the example given in FIG. 3, the gearbox 1 comprises a first planetary gear unit 2a, a second planetary gear unit 2b and a one-stage parallel gear unit 3. Each of the planetary gear units 2a, 2b comprises a planet carrier 4 which supports a plurality of planet gears 5, a ring gear 6 and a sun gear 7. The parallel gear unit 3 of the present example differs from the example shown in FIG. 1 and FIG. 2 in that it only comprises a low speed shaft 8 and a high speed shaft 10, and no intermediate shaft 9. The low speed shaft 8 and the high speed shaft 10 are rotatably supported by bearings 11. In this example, the parallel gear unit 3 comprises one gear 16 which is carried on the low speed shaft 8 and the high speed shaft 10 and which meshes with pinion 17 on the high speed shaft 10.
During operation of the wind turbine, loads acting on shafts and planets and forces originating at tooth contacts are created in the gearbox 1. Uptake of such loads and forces in gearboxes is currently done by using roller bearings. Such bearings are provided at, for example, the high speed shaft of the parallel gear unit 3 of the gearbox 1 and have to take up loads originating from the gearing.
Because noise behaviour is an important parameter in the design of wind turbine gearboxes and especially in multi-megawatt wind turbine gearboxes, gears in the parallel gear unit 3 are normally provided with a helical toothing with a big helix angle. However, because of this big helix angle radial and high axial forces are created which are to be uptaken by the bearings provided on the shafts.
Bearing arrangements of e.g. high speed shafts in parallel gear units can be designed as located or as a combination of located and non-located bearing arrangements. However, there is at least one bearing that takes axial load or a combination of axial and radial loads.
During operation, wind turbines, and especially multi-megawatt wind turbines, create high dynamic forces to and speed variations in the gearbox 1. Because of that, loads and speeds during operation of the gearbox 1 can differ from the design loads and speeds, i.e. from the predicted loads and speeds during design of the gearbox 1, and even reverse loads can occur.
However, for multi-megawatt wind turbines, roller bearings 11 which are available on the market may not be able to take up combined loads from axial forces and radial forces any more. Especially the axial forces can be critical on the roller bearings 11. This dynamic behaviour can lead to bearing damages and consequently to bearing failures.
Equivalent dynamic bearing load is determined by:P≈x·Fr+y·Fa  (1)wherein Fr is the actual radial bearing load, Fa the actual axial bearing load, x the radial load factor for the bearing and y the axial load factor for the bearing. The factors x and y depend on the type of bearings used. In case of, for example, a taper roller bearing x may be 0.4 and y may be between 1.3 and 1.6, which illustrates that the contribution of the axial load to the total load is much higher than that of the radial load.
With increase of transmitted power of multi-megawatt wind turbines and the use of gearing with a big helix angle, bearings are required with high capacity in order to be able to take up the actual loads. A disadvantage of this, however, is that with higher bearing capacity the size of the bearings increases and the very important limiting or maximum speed of the bearing decreases.
Lots of wind turbines have generators running with 1500 up to 2000 rpm what is their maximum speed and therefore the nominal speed of the gearbox shaft. While having combined forces out of radial and axial loads, the resulting load can be higher than the radial load. In these cases, no suitable roller bearings can be found to take up the loads under the required speed.